System, Apparatus, and Method for Tracking Atmospheric Differential Absorption

ABSTRACT

A system, apparatus, and method is provided to remotely measure atmospheric species using a long path differential absorption technique. In one embodiment, a source and a detector are collocated and at the far end of the absorption path a retro-reflector is mounted on a vehicle. The source generates an outgoing laser beam that is transmitted to the retro-reflector and reflected towards the detector as an incoming laser beam, and the detector receives the incoming laser beam that was reflected by the retro-reflector.

FIELD

The present invention relates to tracking atmospheric differentialabsorptions and, more particularly, to a system, apparatus, and methodfor measuring concentrations of atmospheric species.

BACKGROUND

Measuring and identifying species in the atmosphere is critical fornumerous applications ranging from climate science, air quality andpollutant identification, chemical or petroleum plant monitoring, tochemical weapons identification. Accordingly, it may be beneficial tomore accurately and flexibly measure such species remotely.

SUMMARY

Certain embodiments of the present invention may provide solutions tothe problems and needs in the art that have not yet been fullyidentified, appreciated, or solved by current species measurementsystems and methods. For example, the embodiments described hereinprovide a new means of remotely measuring atmospheric species using along path differential absorption technique, where a source and adetector are collocated and at the far end of the absorption path, aretro-reflector is mounted on a vehicle.

In accordance with an embodiment of the present invention, an apparatusis provided. The apparatus includes a source and a detector. The sourcegenerates an outgoing laser beam that is transmitted to one or moreretro-reflectors and reflected towards the detector as an incoming laserbeam. The detector then receives the incoming laser beam that wasreflected by the one or more retro-reflectors.

In another embodiment of the present invention, a method is provided.The method includes generating, at a source, an outgoing laser beam thatis transmitted to one or more retro-reflectors and reflected towards thedetector as an incoming laser beam, and receiving, at the detector, theincoming laser beam that was reflected by the one or moreretro-reflectors.

In yet another embodiment of the present invention, an apparatus isprovided. The apparatus includes a plurality of laser units, atransceiver, and a plurality of detectors. The plurality of laser unitscan generate at least a first wavelength and a second wavelength for aspecie of interest. The transceiver transmits a laser beam to one ormore retro-reflectors, and receives a reflected laser beam from the oneor more retro-reflectors. The plurality of detectors can receive atleast a first reflected wavelength and a second reflected wavelength.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the advantages of certain embodiments of the inventionwill be readily understood, a more particular description of theinvention briefly described above will be rendered by reference tospecific embodiments that are illustrated in the appended drawings.While it should be understood that these drawings depict only typicalembodiments of the invention and are not therefore to be considered tobe limiting of its scope, the invention will be described and explainedwith additional specificity and detail through the use of theaccompanying drawings, in which:

FIG. 1 illustrates a system for tracking atmospheric differentialabsorption, in accordance with an embodiment of the present invention.

FIG. 2 illustrates a system for tracking atmospheric differentialabsorption, in accordance with an embodiment of the present invention.

FIG. 3 illustrates a method for tracking atmospheric differentialabsorption, in accordance with an embodiment of the present invention.

FIG. 4 illustrates a system for tracking atmospheric differentialabsorption, in accordance with an embodiment of the present invention.

FIG. 5 illustrates a system for tracking atmospheric differentialabsorption, in accordance with an embodiment of the present invention.

FIG. 6 illustrates a ground-based laser system, in accordance with anembodiment of the present invention.

FIG. 7 illustrates a ground-based laser system, in accordance with anembodiment of the present invention.

FIG. 8 illustrates a ground-based laser system, in accordance with anembodiment of the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

It will be readily understood that the components of the invention, asgenerally described and illustrated in the figures herein, may bearranged and designed in a wide variety of different configurations.Thus, the following detailed description of the embodiments is notintended to limit the scope of the invention as claimed, but is merelyrepresentative of selected embodiments of the invention.

The features, structures, or characteristics of the invention describedthroughout this specification may be combined in any suitable manner inone or more embodiments. For example, the usage of “certainembodiments,” “some embodiments,” or other similar language, throughoutthis specification refers to the fact that a particular feature,structure, or characteristic described in connection with an embodimentmay be included in at least one embodiment of the invention. Thus,appearances of the phrases “in certain embodiments,” “in someembodiments,” “in other embodiments,” or other similar language,throughout this specification do not necessarily all refer to the sameembodiment or group of embodiments, and the described features,structures, or characteristics may be combined in any suitable manner inone or more embodiments.

One or more embodiments described herein pertain to a system, apparatus,and method for remotely measuring atmospheric species using a long pathdifferential absorption technique, where a source and a detector arecollocated and a retro-reflector mounted on a vehicle is at the far endof the absorption path. The vehicle can be a space vehicle (SV), amanned aerial vehicle (MAV), an unmanned aerial vehicle (UAV), or avehicle on the earth's surface (ESV), or any vehicle that would beappreciated by a person of ordinary skill in the art. The MAV can be aconventional piloted aircraft, and the UAV can be a remotely pilotedvehicle (RPV) or an autonomously piloted vehicle (APV).

The vehicle can be moving and its location can be tracked by adifferential absorption transceiver. The transceiver includes one ormore lasers or other light sources collimated into a beam andtransmitted through a set of tracking mirrors to the target vehicle. Thetransceiver further includes one or more detectors to measure one ormore retro-reflected light beam intensities.

In one embodiment, the transceiver (i.e., receiver and transmitter) canbe monostatic and a diplexer can be used to separate the outgoing light(or beam) from the incoming light (or beam). In another embodiment, thetransceiver can be a bistatic system, such that the receiver andtransmitter aperture are placed next to each other but share a commonfar-field of view.

The vehicle can include a retro-reflector, such as a corner cube ofretro-reflective tape, to allow the transmitter to project two or morewavelengths (through a single beam), either in rapid succession orsimultaneously, to the vehicle and allow return signals to be detectedat the receiver. It should be appreciated that the plurality ofwavelengths can be projected and received simultaneously by the system.Such a configuration reduces perturbations due to atmospheric turbulenceand other effects that reduce the sensitivity of the technique.

In an alternative embodiment, the transmitter can project a plurality ofbeams in a time multiplexed fashion to a plurality of vehicles. Such anembodiment allows the plurality of vehicles to be in the air at the sametime at different locations and be addressed in rapid succession by thesystem. This embodiment also allows a larger space of coverage tomeasure species in shorter periods of time.

FIG. 1 illustrates a system 100 for tracking atmospheric differentialabsorption, in accordance with an embodiment of the present invention.System 100 includes a ground-based laser unit (or transceiver) 102 and avehicle 104. Vehicle 104 can be a SV, MAV, UAV, or an ESV and includes aretro-reflector (or corner cube) 106, bright tracking light-emittingdiode (LED) 108, and a global positioning system (GPS) receiver 110. Inan alternative embodiment, LED 108 can be a beacon or any device thatwould be readily appreciated by a person of ordinary skill in the art.The ground-based laser unit may be deployed as either a stationary,portable, or mobile unit, or can be deployed on an aerial vehicle.

In this embodiment, transceiver 102 monitors the round-trip intensityand/or phase differential(s) between at least two different wavelengths.For example, the detection of a single specie is possible when twowavelengths are monitored. In this case, one laser wavelength may betuned on (X.) and the other laser wavelength may be tuned off (λ_(off))the narrow wavelength region of a particular absorption feature (orresonance absorption spectral feature). In some embodiments, two or morelaser wavelengths are tuned to positions within the wavelength limits ofa particular absorption feature. These wavelengths are chosen such thatthe molecule possesses different absorption strengths for at least twoof the laser wavelengths employed.

In order to monitor the round-trip intensity differential, transceiver102 transmits a beam having at least two different wavelengths throughan air column (or target of interest) to retro-reflector 106, andreceives a return laser beam from retro-reflector 106. The difference inthe measured round-trip intensities of the different wavelengthsmonitored includes information on the integrated-path concentration ofone or more species of interest.

In order to monitor the round-trip phase differential, transceiver 102transmits a beam having at least two different wavelengths through anair column (or target of interest) to retro-reflector 106, and receivesa return laser beam from retro-reflector 106. The laser wavelengths areagain selected to sample portions of the absorption feature of themolecule to be detected. The phase of each wavelength is measured byheterodyning the return laser beam with a reference signal derived fromthe outgoing beam of the same wavelength. The beat signals are recorded.The phase of each wavelength beam signal is unwrapped from the beatsignals and the difference in the phases of the wavelength signalsincludes information on the integrated-path concentration of one or morespecies of interest.

Extraneous sources of phase difference due to things such as platformmotion can be common between the different wavelengths used and can beremoved by differencing the phase signals of the different wavelengthsused. The determination of the integrated-path concentrations of nspecies of interest over the system's optical path from the transceiverto the retro-reflector and back to the transceiver may require themeasurement of the round-trip intensities and/or phase differentials atm>=n different monitoring wavelengths, where m is the number ofwavelengths and n is the number of species.

It should be appreciated that vehicle 104 can be actively tracked by thereceiver system using several methods, including mounting a modulatedbeam (e.g., LED 108) on vehicle 104 or simply closing a tracking loop onthe returned laser beam. In one embodiment, the location of vehicle 104can be determined using a time-tagged GPS receiver 110 mounted onvehicle 104, and downloaded either post-flight or by telemetry. Inanother embodiment, tracking gimbal mount angles and some type of laserranging method are utilized to determine the position of vehicle 104 inthree dimensions.

FIG. 2 illustrates a system 200 for tracking atmospheric differentialabsorption, in accordance with an embodiment of the present invention.System 200 includes a ground-based laser system 202 and a vehicle 210.Laser system 202 includes a laser unit (or source) 204, an opticaltransceiver 206, and a pointing and tracking optics unit 208. Vehicle210 includes a retro-reflector 212 and a tracking beacon 214.

In this embodiment, laser unit 204 is capable of being tuned to generatem different wavelengths, where m>=2 for n species or molecules ofinterest where n>=1 and m>=n. In a case where two monitoring wavelengthsare used, the first wavelength can be configured to be on an absorptionfeature of a specie of interest and the second wavelength can beconfigured to be off of the absorption feature of the specie ofinterest. For example, if the specie of interest is carbon dioxide(CO₂), then the first wavelength is configured to be on resonance withrotational-vibrational transitions of CO₂ and the second wavelength isconfigured to be off of the resonance.

The wavelengths can be generated simultaneously or successively andcombined using, for example, a multiplexer (MUX) to form a laser beam.If the wavelengths are generated successively, then the secondwavelength may be generated less than one millisecond after the firstwavelength is generated. This allows system 200 to reduce errorsassociated with changing atmospheric or platform conditions.

The laser beam passes through an optical transceiver 206 such that laserbeams can be transmitted to, and received from, retro-reflector 212attached to vehicle 210. Optical transceiver 206 can include a devicesuch as a polarization splitter to distinguish between a transmittedlaser beam and a received laser beam based on the polarization of thetransmitted and received laser beams. This allows optical transceiver206 to transmit and receive simultaneously. In another embodiment, thetransmitted and received beams are separated using an optical circulatordevice. In the case where the wavelengths are combined into a singletransmit beam by wavelength multiplexing, the wavelengths of thereceived beam may be separated according to their wavelength using awavelength de-multiplexor known as a DEMUX. Other wavelength separationmethods such as dichroic mirrors or dispersive elements such asdiffraction gratings could also be used to separate the receivedwavelengths. However, this is a matter of design choice.

The laser beam that is being transmitted to vehicle 210 passes through apointing and tracking optics unit 208. This allows the laser beam to betransmitted to retro-reflector 212, which is attached to a movingvehicle. In order to accurately point the laser beam to retro-reflector212, pointing and tracking optics unit 208 utilizes, for example,movable mirrors, a gimbal, or a coherent array of laser beams, which arethen phased appropriately to track an angle. However, it should beappreciated that pointing and tracking optics unit 208 can utilize anydevice that can accurately point the laser beam to the target in space,air, or on the earth's surface.

The laser beam then propagates out of the transmitter through thespecies (or gas) of interest to retro-reflector 212, and returns topointing and tracking optics unit 208 along or almost along the samepath of travel. It should be appreciated that retro-reflector 212 can bea corner cube that allows the laser beam to be sent along the sameangular path on which the laser beam traversed. In another embodiment,retro-reflector 212 may be a mirror, reflective paint, or reflectivetape that allows the laser beam to bounce back to ground-based lasersystem 202. Pointing and tracking optics unit 208 is then configured todirect the received laser beam to transceiver 206 for processing.

Ground-based laser system 202 is also configured to track the movementof vehicle 210 in order to accurately align the laser beam withretro-reflector 212. In order to accomplish this tracking, vehicle 210is equipped with a tracking beacon 214 that transmits a beacon signal topointing and tracking optics unit 208. Pointing and tracking optics unit208 may be equipped with an imaging device (not shown) that can move thepointer in pointing and tracking optics unit 208 until the beacon signalis centered on a focal plane. Once the beacon signal is centered on thefocal plane, the pointer can automatically align the transmitted laserbeam with retro-reflector 212. In order to track vehicle 210, a controlloop can be utilized to keep track of, and maintain view of, vehicle 210using at least the focal plane and pointing and tracking optics unit208.

FIG. 3 illustrates a method 300 for tracking atmospheric differentialabsorption, in accordance with an embodiment of the present invention.Method 300 can be executed by the ground-based laser system describedin, for example, FIG. 2. However, it should be appreciated that theother embodiments described herein may also perform the methodillustrated in FIG. 3.

In this embodiment, a laser unit (or source) generates at 302 aplurality of wavelengths for the specie of interest. At least two of thewavelengths employed are configured to sample absorption strengthdifferences in the species of interest. For example, one of thewavelengths may be configured to be on an absorption feature of a specieof interest and another wavelength may be configured to be off of theabsorption feature of the specie of interest. At 304, the generatedwavelengths are combined into a single laser beam. The laser beam istransmitted at 306 through an air column containing the specie ofinterest to a retro-reflector of a vehicle. At 308, the transmittedlaser beam reflects off of the retro-reflector and is received by adetector. At 310, the detector measures the intensity and/or phase ofthe wavelengths contained in the reflected laser beam for the specie ofinterest.

The method steps shown in FIG. 3 may be performed, in part, by acomputer program, encoding instructions for a processor to cause atleast the methods described in FIG. 3 to be performed by the apparatusesdiscussed herein. The computer program may be embodied on anon-transitory computer readable medium. The computer readable mediummay be, but is not limited to, a hard disk drive, a flash device, arandom access memory, a tape, or any other such medium used to storedata. The computer program may include encoded instructions forcontrolling the processor to implement the method described in FIG. 3,which may also be stored on the computer readable medium.

The computer program can be implemented in hardware, software, or ahybrid implementation. The computer program can be composed of modulesthat are in operative communication with one another, and which aredesigned to pass information or instructions to display. The computerprogram can be configured to operate on a general purpose computer, oran application specific integrated circuit (“ASIC”).

FIG. 4 illustrates a system 400 for tracking atmospheric differentialabsorption, in accordance with an embodiment of the present invention.System 400 includes a ground-based laser system 402 and a vehicle 408.Ground-based laser system 402 includes a transmitter 404 and a pointingand tracking optics unit 406. Vehicle 408 includes a detector 410 and atracking beacon 412.

In this embodiment, ground-based laser system 402 generates a laser beamcontaining m different wavelengths (where m>=2) for the n differentspecies of interest (where m>=n), and transmits, using transmitter 404,the laser beam to detector 410 attached to vehicle 408. By transmittingthe laser beam to detector 410, data can be recorded and processed onvehicle 408. For example, vehicle 408 can measure the intensities of thewavelengths for the species (molecular or chemical species) of interest.

FIG. 5 illustrates a system 500 for tracking atmospheric differentialabsorption, in accordance with an embodiment of the present invention.In this embodiment, system 500 includes a ground-based laser system 502,which can be a ground-based site. In a simple example, in order togenerate two wavelengths for the detection of a specie of interest(e.g., a λ_(A1on) resonance and a λ_(A2off) resonance for specie A), afirst laser unit (or seed laser unit) 504 generates the first wavelength(λ_(A1on)) and a second laser unit (or seed laser unit) 506 generatesthe second wavelength (λ_(A2off)) for detection of specie A. Seed laserunits 504 and 506 (or laser sources) can be narrow wavelengthsemiconductor laser units, which operate at a specific wavelength in theinfrared range (e.g., 1.0 microns to 12.5 microns in wavelength). Itshould be appreciated that system 500 may include more than two seedlaser units, and a plurality of wavelengths can be generated for eachspecies of interest. For instance, it should be noted that for every nspecies, there can be m wavelengths, where m>n or m=n.

In the example given above, the two wavelengths can be then combinedusing, for example, a dense wavelength division multiplexor (MUX) 508 togenerate or form a single laser beam. The generated laser beam istransmitted through a fiber amplifier (optical amplifier) 510 and isamplified in order to transmit the laser beam to a retro-reflector.Ground-based laser system 502 utilizes an isolator 512 to ensure thatreflections are not transmitted to fiber amplifier 510, and utilizes acollimator 514 to collimate the amplified laser beam into a laser beamthat does not have too much divergence such that a useful fraction ofthe laser energy is reflected back from the remote retro-reflector.

The laser beam also passes through a polarization beam splitter (orpolarization unit) 516 to differentiate the laser beam (e.g., outgoinglaser beam) from the incoming (or reflected) laser beam. Because theoutgoing laser beam and the incoming laser beam are polarized,polarization unit 516 allows the light with one polarization to pass,but reflects the light with another polarization 90 degrees. This causespolarized light from the incoming laser beam that was reflected 90degrees to be redirected to a dense wavelength division demultiplexer(or DEMUX) 526. More specifically, light to be transmitted to vehicle536 (outgoing laser beam) is not reflected 90 degrees, but light thatwas reflected off of, and received from, vehicle 536 (incoming laserbeam) is reflected 90 degrees.

The outgoing laser beam then passes through, for example, a quarter-waveplate 518 that converts vertically plane polarized light into circularpolarized light. It should be appreciated that in this embodiment, aquarter-wave plate is utilized, but in other embodiments a differenttype of wave plate may be utilized or no wave plate may be utilized, asthis is a matter of design choice. The outgoing laser beam, whichincludes circular polarized light, passes through beam splitter 520 andtelescope 522, and is directed to the retro-reflector attached to avehicle 536, via a pointer 524. Pointer 524 can point the laser beam intwo dimensions to essentially cover a hemisphere of space, or even thewhole sphere if the transmitter and the pointer are on an aerialvehicle. Pointer 524 can also be configured to track theretro-reflector. In other words, pointer 524 is a tracking device and atransmitting device.

The outgoing laser beam passes through a column of air, reflects off ofthe retro-reflector, and is transmitted back to pointer 524 along thesame or a similar angular path. Pointer 524 directs the reflected laserbeam to polarization unit 516. In particular, pointer 524 directs thereflected laser beam to wave plate 518 so the reflected laser beam canbe rotated 90 degrees and, thus, become plane polarized light. Becausethe reflected laser beam is rotated 90 degrees, polarization unit 516,which receives the rotated laser beam, can distinguish between theoutgoing and reflected laser beams. This allows polarization unit 516 toseparate the outgoing laser beam from the reflected laser beam (orincoming laser beam), and direct the incoming laser beam to DEMUX 526.

DEMUX 526 separates the wavelengths contained in the reflected laserbeam into two separate wavelengths, and transmits the separatedwavelengths to a first detector 528 and a second detector 530. By havingat least two detectors, the two wavelengths can be sampled at the sametime. In other words, the detectors allow the intensity between the tworeturned wavelengths to be analyzed and compared with the twotransmitted wavelengths, and also provide the integrated-pathconcentration of the specie being measured.

In order to accomplish tracking of the retro-reflector on vehicle 536, afocal plane method is used in this embodiment. For instance, a beacon,such as an LED that emits 800 nm light, is mounted on vehicle 536. TheLED is configured to shine a broad beam of light (or beacon signal) topointer 524, which directs the beam of light to a dichroic beam splitter520. Beam splitter 520 can direct the beam of light to a tracking focalplane 534, via bandpass filter 532 that transmits 800 nm light. Thebandpass transmission filter 532 is configured to filter out anybackground light of wavelengths other than 800 nm. The bandpass filter'stransmission wavelength is chosen to match the wavelength of lightemitted by the beacon. The beacon/filter wavelength employed can be ofwavelength regions other than 800 nm according to design choice. Thisallows tracking focal plane 534 and pointer 524 to constantly align withmoving vehicle 536. For instance, as the LED is moving, pointer 524 ismoved until the laser beam is pointed at vehicle 536. Such methods canalso be used to accomplish tracking of the retro-reflectors separatelylocated on multiple vehicles or aircraft.

In one or more embodiments, the reflected laser signal itself can beused to track the vehicle without an additional LED beacon. In thiscase, the laser pointer is dithered such that the transmitted laser beamsweeps out a sufficiently large solid angle that the laser beamrepetitively illuminates the retro-reflector on the vehicle. Theresulting reflected signal strength is then used as a diagnostic forpointing in a feedback locking loop.

It should also be appreciated that vehicle 536 can also be equipped witha photo-electric device that can extract a portion of energy from thetransmitted laser beam, and store the energy in a battery on vehicle536. This approach can also be used to store energy in batteriesseparately located on several different vehicles.

In one or more embodiments, image processing is used to track vehicle536. Such systems can also be used with multiple vehicles or aircraft.

FIG. 6 illustrates a ground-based laser system 600, in accordance withan embodiment of the present invention. Ground-based laser system 600includes a computer control unit 602, a plurality of seed lasers 604, aswitch or MUX 606, and optical amplifier 608, a splitter (or 1/99splitter) 610, a transmission/reception diplexer (or circulator) 612, aplurality of pointing and tracking optical units 614, a switch or DEMUX616 for a plurality of wavelength detectors 618, and a switch or DEMUX620 for a plurality of reference detectors 622. It should be appreciatedthat the devices included within ground-based laser system 600 may befiber optic devices.

In this embodiment, computer control unit 602 may include a processor,memory, and other electronic devices that would be appreciated by aperson of ordinary skill in the art. Computer control unit 602 isoperatively connected to seed lasers 604, but can also be connected tothe other devices included in ground-based laser system 600. Computercontrol unit 602 can activate and deactivate seed lasers 604, andcontrol each unit or device within ground-based laser system 600. Forexample, computer control unit 602 can select which wavelength (X) oflaser that is to be transmitted at any one time, or can select aplurality, or even all, of the wavelengths of the lasers to betransmitted.

In this embodiment, FIG. 6 illustrates four seed lasers 604. However, itshould be appreciated that ground-based laser system 600 can include anynumber of seed lasers 604. For example, computer control unit 602 canselect all four seed lasers 604 in order to generate four wavelengthsfor detecting two species of interest, or select, for example, two ofthe four seed lasers 604 in order to generate two wavelengths fordetecting a particular specie of interest. It should also be appreciatedthat computer control unit 602 can select one of four seed lasers 604 inFIG. 6 in order to generate a single wavelength to qualitatively assesswhether a single specie of interest might be present.

Once seed lasers 604 generate the wavelengths, the wavelengths aretransmitted through MUX 606. MUX 606 is configured to combine thewavelengths and generate a laser beam containing the wavelengths. In analternative embodiment, a switch can be utilized. If a switch isutilized instead of a MUX, then the switch can select or changewavelengths based on a certain time period. For example, the switch mayallow the first wavelength to pass through the switch at time t₁, thenthe second wavelength at time t₂, the third wavelength at time t₃, andso on.

The generated laser beam also passes through a fiber optical amplifier608. Optical amplifier 608 is configured to amplify one or morewavelengths, or the entire laser beam, in order for the wavelengths orlaser beam to be transmitted to a retro-reflector attached to a vehicle.

Ground-based laser system 600 also uses a 1/99 splitter 610 to split onepercent of power from the laser beam, and transmit the one percent ofthe power to a DEMUX (or switch) 620. However, the amount of power thatis split from the laser beam is a matter of design choice. DEMUX 620separates the wavelengths contained in the laser beam and transmits theseparated wavelengths to a plurality of reference detectors 622.Reference detectors 622 detect the intensity of the wavelengths prior totheir transmission to the retro-reflector. This allows a reference pointto be generated to show how much power is being generated at the outputof system 600, and the generated power level is also recorded incomputer control unit 602.

At the same time, the remainder of power (ninety-nine percent) of thelaser beam is transmitted to a transmission/reception diplexer (orcirculator) 612. Circulator 612 transmits the laser beam through acertain port based on the direction of the laser beam. For example, whenthe laser beam is received at circulator 612 from splitter 610,circulator 612 allows the laser beam to be transmitted to theretro-reflector of the vehicle via pointing and tracking optics unit614.

When the laser beam reflects back from the retro-reflector, thereflected (or received) laser beam is received at circulator 612 viapointing and tracking optics unit 614. Circulator 612 receives thereflected laser beam and directs the reflected laser beam through a portthat allows the reflected laser beam to be transmitted to DEMUX (orswitch) 616. DEMUX 616 separates the light of different wavelengthscontained in the reflected laser beam and transmits thewavelength-separated light to the appropriate wavelength detectors 618.Detectors 618 convert the wavelength-separated light to electrons, whichare counted to determine the intensity of the light at each wavelength.The light intensities at the different wavelengths can then be stored inthe memory of computer control unit 602. This allows computer controlunit 602 to compare the intensities of the transmitted wavelengths tothe reflected wavelengths in order to measure and identify species inthe atmosphere.

FIG. 7 illustrates a ground-based laser system 700, in accordance withan embodiment of the present invention. Ground-based laser system 700may include similar components, which may have similar functionality tothe components described above with respect to FIG. 6. However, in thisembodiment, ground-based laser system 700 pertains to detection of phaserather than intensity change associated with the presence of adifferential absorption of a molecular specie of interest.

For example, ground-based laser system 700 includes a computer controlunit 702, a plurality of seed lasers 704, local oscillators connected tomixers 718 and 724, a MUX 706, an optical amplifier 708, a splitter 710,a circulator 712, pointing and tracking optics 714, DEMUX 716 and 722, aplurality of detectors 720, and a plurality of reference detectors 726.

In this embodiment, seed lasers 704 are tuned to generate light of atleast two different wavelengths that fall within the wavelength limitsof an absorption feature of a species of interest. This specie possessesdifferent absorption strengths at two or more of the wavelengths oflight produced. A small fraction of power (approximately 25 percent) canbe split from each of the generated wavelengths to provide localoscillators for the heterodyne detection for both a reference channeland a signal channel. It should be appreciated that the fraction ofpower being split can be any percentage for the heterodyne detection forboth the reference channel and the signal channel.

For example, if two wavelengths were generated for a given specie ofinterest, then approximately 25 percent of power from each of the firstand second wavelengths can be passed to mixers 718 and 724. Theremaining power for each of the first and second wavelengths(approximately 75 percent) is then combined into a single beam using MUX706 or a series of dichroic mirrors (not shown). The combinedwavelengths are amplified in a single optical amplifier 708 havingenough gain bandwidth to amplify all wavelengths.

Splitter 710 can then split a small portion of power (approximately 1percent) of the amplified beam to serve as a reference beam. Thisreference beam is passed through DEMUX 722 and is separated into atleast two individual wavelength references, e.g., a first individualwavelength reference and a second individual wavelength reference.Mixers 724 are configured to mix each individual wavelength reference,for example on a photodiode, with the appropriate local oscillator ofthe same wavelength. It should be appreciated that the local oscillatorand reference signal may include the same polarization and spatial modestructure in order to allow efficient mixing on the photodiode. As aresult, the heterodyned signal can be digitized and stored in a computer(e.g., computer control unit 702) as a phase reference.

A major fraction of power (approximately 99 percent) of the amplifiedbeam is sent through a circulator 712 or similar device to separate thetransmitted beam from a received beam. The transmitted beam is thendirected to a retro-reflector attached on, for example, a movingvehicle. The beam is returned along the same direction, and is directedalong the same path, as the transmitted beam, but travels in theopposite direction. The returned (or reflected) beam passes throughcirculator 712 and is directed to DEMUX 716, which separates thereturned beam into at least two reflected wavelengths, a first reflectedwavelength and a second reflected wavelength. Each reflected wavelengthis then mixed using mixers 718, for example on a photodiode, with theappropriate local oscillator of the same wavelength to generate aheterodyned signal. This heterodyned signal can then be digitized andstored in a computer.

The phase is then unwrapped from the heterodyned signal and comparedwith the phase from the appropriate reference channel to determine aphase difference. The phase difference measured for a more stronglyabsorbed wavelength and the phase difference measured for a lessstrongly absorbed wavelength can then be differenced and that differencecan be directly related to the concentration of the specie of interestalong the beam path.

Stated another way, to measure the phase difference (or first phasedifference) of the first wavelength, the phase before the firstwavelength is transmitted is measured and the phase when the firstwavelength is returned from the retro-reflector is measured. Similarlyto measure the phase difference (or second phase difference) of thesecond wavelength, the phase before the second wavelength is transmittedis measured and the phase when the second wavelength is returned fromthe retro-reflector is measured. From these differences, the first phasedifference and the second phase difference are compared to determine thephase difference of a specie of interest.

FIG. 8 illustrates a ground-based laser system 800, in accordance withan embodiment of the present invention. Ground-based laser system 800may include similar components to FIG. 6, and may have similarfunctionalities as described above with respect to FIG. 6. For example,ground-based laser system 800 includes a computer control unit 802, aplurality of seed lasers 804, at least two MUX 806 and 818, an opticalamplifier 808, at least two splitters 810 and 820, a circulator (ordiplexor) 812, pointing and tracking optics 814, frequency shifters 816,at least two mixers 822 and 824, a reference detector 826, and a signaldetector 828.

In this embodiment, ground-based laser system 800 measures phasedifferences rather than intensity differences to determine the molecularpath concentration of the species of interest. Let's assume for a singlespecie of interest, at least two wavelengths of light are generated, afirst wavelength and a second wavelength, by at least two of seed lasers804. Using local oscillators, a small fraction of power is split offfrom the first wavelength and transmitted to MUX 818, and a smallfraction of the wavelength is split off from the second wavelength andtransmitted to MUX 818. This small fraction of power can beapproximately 5 percent, 10 percent, 25 percent, or any percentage thatwould be appreciated by a person of ordinary skill in the art. MUX 818then combines the small fraction of power for each of the wavelengthsthat were split off into a single beam and transmits the single beam tosplitter 820. Splitter 820 splits half of the power of the single beamand transmits half of the power of the single beam to mixer 822.Splitter 820 transmits the other half of the power to mixer 824 to becombined with a returned beam.

The first and second wavelength can then pass through frequency shifters816, and the first wavelength (λ₁) is modulated with a frequency (Δ₁) togenerate a first modulated wavelength (λ₁+1/Δ₁) and the secondwavelength (λ₂) is modulated with a frequency (Δ₂) to generate a secondmodulated wavelength (λ₂+1/Δ₂). In this embodiment, for each wavelengthbeing generated, frequency shifters 816, such as acousto-opticmodulators, are utilized to attach a frequency-generated offset to aparticular wavelength. This allows detectors 826 and 828 to distinguishbetween the wavelengths based on the frequency-generated offset beingattached to each wavelength. This also eliminates the need for a DEMUXon the receiving side of system 800.

The first modulated wavelength and the second modulated wavelength arethen combined into a single modulated beam using a MUX 806, and thesingle modulated beam is amplified using an optical amplifier 808.Splitter 810 then splits off a small fraction of power (e.g.,approximately 1 percent) from the amplified beam, and transmits thesmall fraction of power of the amplified beam to mixer 822. Mixer 822combines the small fraction of the amplified beam with the beam that wassplit by splitter 820 in order to generate a reference signal for thespecie of interest and transmits this reference signal to detector 826.This reference signal can then be stored and digitized as a phasereference.

Mixer 824 can receive and mix a returned beam from a retro-reflectorwith the beam that was split by splitter 820 to generate a mixed signalto be transmitted to detector 828. It should be appreciated that ratherthan separate the received channels by wavelength and then mix with alocal oscillator, this embodiment utilizes mixer 824 to mix the entirereceived beam on a single detector 828 with a multiplexed localoscillator containing all of the wavelengths as the entire referencebeam. When the signal and reference beams are mixed with the localoscillator, each channel has a different intermediate frequency (IF)since each channel was frequency shifted from its local oscillatorsfrequency by a different delta frequency. IF filters are used in thisembodiment to eliminate unwanted IF products. To measure the phasedifference, a similar technique can be employed as described above withrespect to FIG. 7.

The embodiments described herein provide several advantages over currentmethods and systems. For example, the embodiments allow the targetreflector to move, especially in the air, allow the path to be measured,and allow the path to be arbitrarily adjusted at the will of theoperator. Also, any clear line-of-sight path could be investigated andadjusted with ease. For example, a factory stack plume could beinterrogated at several locations along the plume.

Another advantage provided by the embodiments is that an up-looking pathcan easily be investigated by a vehicle above a hill or building. A twoor three-dimensional map of species concentrations in the interveningpath can be developed by driving or flying the vehicle in a desiredpattern across the area of interest.

Also, by having the transmitter and receiver collocated, most of thecomplexity resides in the fixed ground site and the vehicle can be verysimple, small, and inexpensive. This would allow for the loss of avehicle to be of less concern in the event that it becomes contaminatedor damaged during its deployment. For example, if the vehicle were flownover a potentially high concentration of radioactive particles, then thevehicle can be disposed of remotely, without the concern of havingexpensive equipment on board that must be replaced.

The fixed site could accommodate a wide range of lasers and detectors,with less concern regarding the size, weight, power, cost, androbustness to shock, vibration and other environmental parametersrequired to mount the transceiver itself on the vehicle.

One having ordinary skill in the art will readily understand that theinvention as discussed above may be practiced with steps in a differentorder, and/or with hardware elements in configurations that aredifferent than those which are disclosed. Therefore, although theinvention has been described based upon these preferred embodiments, itwould be apparent to those of skill in the art that certainmodifications, variations, and alternative constructions would beapparent, while remaining within the spirit and scope of the invention.In order to determine the metes and bounds of the invention, therefore,reference should be made to the appended claims.

1. An apparatus, comprising: a source configured to generate an outgoinglaser beam that is transmitted to one or more retro-reflectors; and adetector configured to receive an incoming laser beam that is reflectedfrom the one or more retro-reflectors.
 2. The apparatus of claim 1,wherein the outgoing laser beam comprises a first wavelength and asecond wavelength configured to measure different absorption strengths.3. The apparatus of claim 1, wherein the source comprises a plurality oflaser units configured to simultaneously or successively generate atleast a first wavelength and a second wavelength for measuring differentabsorption strengths of a specie of interest.
 4. The apparatus of claim1, wherein the source comprises a plurality of laser units configured togenerate m wavelengths for measuring different absorption strengths of nspecies, where m is n, or m is another number greater than n.
 5. Theapparatus of claim 1, further comprising: a reference detectorconfigured to measure the intensities of at least a first wavelength anda second wavelength of the outgoing laser beam before the outgoing laserbeam is transmitted to the one or more retro-reflectors.
 6. Theapparatus of claim 1, further comprising: a pointer configured to alignand direct the outgoing laser beam to the one or more retro-reflectorsand direct the incoming laser beam to a polarization unit.
 7. Theapparatus of claim 6, wherein the polarization unit is configured todistinguish between the outgoing laser beam and the incoming laser beam,direct the outgoing laser beam from the source to the pointer, anddirect the incoming laser beam from the pointer to the detector.
 8. Theapparatus of claim 6, further comprising: a wave plate configured torotate the incoming laser beam 90 degrees to form plane polarized lightsuch that the polarization unit can distinguish between the outgoinglaser beam and the incoming laser beam.
 9. The apparatus of claim 1,wherein the incoming laser beam comprises at least a first reflectedwavelength and a second reflected wavelength.
 10. The apparatus of claim9, wherein the detector is further configured to measure the intensitiesof at least the first reflected wavelength and the second reflectedwavelength of the incoming laser beam.
 11. The apparatus of claim 1,further comprising: a control unit configured to measure at least afirst difference in phase between a first wavelength and a firstreflected wavelength, measure at least a second difference in phasebetween a second wavelength and a second reflected wavelength, andcompare differences between at least the first difference in phase andthe second difference in phase to determine a concentration of a specieof interest.
 12. A method, comprising: generating, at a source, anoutgoing laser beam that is transmitted to one or more retro-reflectors;and receiving, at a detector, an incoming laser beam that is reflectedfrom the one or more retro-reflectors.
 13. The method of claim 12,wherein the outgoing laser beam comprises at least a first wavelengthand a second wavelength configured to measure different absorptionstrengths.
 14. The method of claim 12, further comprising:simultaneously or successively generating, at the source, at least afirst wavelength and a second wavelength for a specie of interest; andcombining at least the first and second wavelength to produce theoutgoing laser beam.
 15. The method of claim 12, further comprising:generating m wavelengths for measuring different absorption strengths ofn species, where the m wavelengths is n, or m is another number greaterthan n.
 16. The method of claim 12, further comprising: directing, by apointer, the outgoing laser beam to the one or more retro-reflectors anddirecting the incoming laser beam to a polarization unit.
 17. The methodof claim 16, further comprising: rotating, at a wave plate, the incominglaser beam 90 degrees to form plane polarized light such that thepolarization unit can distinguish between the outgoing laser beam andthe incoming laser beam.
 18. The method of claim 17, further comprising:distinguishing, at the polarization unit, between the outgoing laserbeam and the incoming laser beam, directing the outgoing laser beam fromthe source to the pointer, and directing the incoming laser beam fromthe pointer to the detector.
 19. The method of claim 12, wherein theincoming laser beam comprises at least a first reflected wavelength anda second reflected wavelength.
 20. The method of claim 19, furthercomprising: measuring, at least a first detector and at a seconddetector, the intensities of at least the first reflected wavelength andthe second reflected wavelength of the incoming laser beam.
 21. Themethod of claim 12, further comprising: measuring the intensities of atleast a first wavelength and a second wavelength of the outgoing laserbeam before the outgoing laser beam is transmitted to the one or moreretro-reflectors.
 22. The method of claim 12, further comprising:measuring at least a first difference in phase between a firstwavelength and a first reflected wavelength; measuring at least a seconddifference in phase between a second wavelength and a second reflectedwavelength; and comparing differences between at least the firstdifference and the second difference to determine a concentration in aspecie of interest.
 23. An apparatus, comprising: a plurality of laserunits configured to generate at least a first wavelength and a secondwavelength for a specie of interest; a transceiver configured totransmit a laser beam to one or more retro-reflectors, and receive areflected laser beam from the one or more retro-reflectors; and aplurality of detectors configured to receive at least a first reflectedwavelength and a second reflected wavelength.
 24. The apparatus of claim23, wherein the laser beam comprises at least the first wavelength andthe second wavelength, and the reflected laser beam comprises at leastthe first reflected wavelength and the second reflected wavelength. 25.The apparatus of claim 24, further comprising: a multiplexer configuredto combine at least the first wavelength and the second wavelength toform the laser beam; and a demultiplexer configured to separate at leastthe first reflected wavelength and the second reflected wavelength. 26.The apparatus of claim 23, further comprising: a pointer configured todirect the laser beam to the one or more retro-reflectors and receiveand redirect the reflected laser beam from the one or moreretro-reflectors.
 27. The apparatus of claim 23, further comprising: areference detector configured to measure the intensities of at least thefirst wavelength and the second wavelength before the laser beam istransmitted to the one or more retro-reflectors.
 28. The apparatus ofclaim 23, wherein the plurality of detectors are further configured tomeasure the intensities of at least the first reflected wavelength andthe second reflected wavelength for the specie of interest.
 29. Theapparatus of claim 23, further comprising: a control unit configured tomeasure at least a first difference in phase between the firstwavelength and the first reflected wavelength, measure at least a seconddifference in phase between the second wavelength and the secondreflected wavelength, and compare differences between at least the firstand second difference to determine a concentration of the specie ofinterest.
 30. The apparatus of claim 23, further comprising: a controlunit configured to measure differences in phase between an mthwavelength of the transmitted laser beam before the transmitted laserbeam is transmitted to the one or more retro-reflectors and acorresponding mth reflected wavelength of the reflected laser beam; andcompare differences in phase between outgoing and incoming wavelengthsfor the m wavelengths used to determine concentrations of n differentspecies of interest, where m is n, or m is another number greater thann.